Regular Nixie Tube Clock

[cprobertson1]

Yet another of those pesky nixie clocks.

Is mine different? Not really - I could hijack somebody else's post - but tbh then I can't show of sweet pics of my project...

"Sweet pics"?  - okay I won't ever say that again...


This is the counterpart, the contra medium of my "Nixie Clock... Without the nixies!" - which is effectively the same project, but using DIY nixie tube replacements because it seemed like a fun idea to make something easy-to-use.


Well, to make thing short - I have 8x IN-14 nixie tubes to drive from an AVR ATMega168-20PU (which is totally overkill, but it's what I have around and I can't be bothered picking up something else - it also means I have all the IO I could ever wish for)

My plan is to drive the nixies directly, using off-the-shelf parts: I want everything to be easily replaceable and readily available.

I'm thinking:
uC > 8x 16-Bit SIPO Shift Registers^[1] > Optocoupler^[2] > Transistor^[3] > NIXIE

^[1]: each consisting of 2 serial 8-bit shift registers - all driven from the same clock/reset lines - but each group with it's own data line (10 I/O pins in total)

^[2]: Which type of coupler do I want here? Will an optotransistor allow me to avoid having to use a high voltage transistor to drive the nixie cathode?

^[3]: BJT or MOSFET with a high voltage rating - all tied to Vcc/GND - acting as simple switches to power the cathode.

NB - a separate Optocoupler > Transistor will be used to open the Nixie's anode while we're shifting data into the registers (i.e between digit-changes) (8 I/O pins in total)


This brings us to 18 I/O Pins in use, which leaves plenty for our I²C bus (for the RTC, GPS clock and whatever else I decide to add to it in the future) (ATMega168 has 23 I/O - including the reset pin, so 22 I/O just to be safe)

[deephaven]

You can get Russian made  74141 chips which are binary to decimal decoders with built-in high voltage drivers. Then you can drive them with SIPO shift registers with output registers so that the outputs don't see the shifting data - it just gets broadside loaded after the shift in has finished. Also, no real need for an opto-coupler.

[cprobertson1]

Oops - sorry! Was editing my original rambling post when you replied!

I'm trying to use off-the-shelf parts if I can help it =/ Was trying to shy away from the vintage chips (which is a pity, because it'd make it so much easier!)

Is there a modern equivalent to the 74141 chips or have they all gone low-voltage nowadays ?

--EDIT--
Optocouplers aren't particularly cheap when you need a lot of them - even when you get arrayed versions xD Don't suppose driving High Voltage Transistor (BJT/MOSFET) directly via a series diode is an option?

[Ian.M]

Once you get up above about 70V there aren't many integrated solutions still commonly available.  I'd look at doing something like this x the number of electrodes you need to control with discretes in SMD. 

  o-----------------o-----o-----o
  +HT               |     |
                   .-.    |
                   | |    |
                   | |    |
                   '-'    |
                    |   |/  Q2 HV NPN
                    o---|
                    |   |>
                    |     | To Tube
                    |     o-----o
                    |     |
                    |     V  D1
                    |     -
                    |     |
  Logic             o-----'
  Signal            |     
      ___         |/  Q1 HV NPN
  o--|___|---o----|
             |    |>
             |      |
            .-.     |
            | |     |
            | |     |
            '-'     |
  0V         |      |
  o----------o------o-----------o
(created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)

Although you could use a MOSFET for Q1 and omit the base resistors, if something goes wrong and Q1 fails, a bipolar is far less likely to kill your drive logic.  If you are paranoid, add clamping at the logic input!

The pullup resistor should be capable of handling the dissipation with Q1 on all the time. It should also be sized to provide enough base current to Q2 without excessive drop assuming worst case Hfe and max load current.  The transistors and diode should have a voltage rating at least 50% greater than +HT

It shouldn't be too tedious to assemble if you get a PCB + stencil made and have reflow capability.

[deephaven]

Take a look at this range of chips http://www.microchip.com/wwwproducts/Devices.aspx?product=HV6810

These will do your shifting, latching and high voltage driving all in one go. It doesn't look like a they do a very high voltage, but they should work due to the volts drop across the nixie.

[kmm]

I used an HV5122 32 channel open drain driver in my Nixie clock, worked fine.

http://ww1.microchip.com/downloads/en/DeviceDoc/HV5122%20B072213.pdf

32 channels will make a 4 digit clock, with the caveat you only get numbers 1 and 2 on the left most digit. You can easily cascade a similar chip on for another N channels though, or add another discrete driver channel to control the missing leading zero. Two '5122s cascaded will drive a 6 digit clock with a few channels left over.

The '5122 does require a >10V low voltage supply with similar logic levels, so a level shifter will be needed between your digital stuff and it. I just used the simple two-resistors-and-a-mosfet approach; it's only 4 signal lines so no big deal.

[T3sl4co1l]

No need for so many components, a nixie only needs an active pull-down (BJT or MOSFET), and maybe a high ohm pull-up to counter leakage.  The switching device doesn't need to handle all 170V+, but only the ~50V change between "on" and "off" states.  The rest is taken up by the current limiting resistor.

You could arrange common-(emitter/source) or common-(base/gate) switching (in the latter case: B/G to VCC via resistor, MCU pin to E/S with optional current limiting resistor).

Whichever permutation you use, I wouldn't consider it a risk to the MCU.  The HV supply should be current limited, both by brute force (the resistor), and by design (you only need a few mA, so why make a supply capable of piles of current?).  MCU pins are more than capable of sinking this kind of current, in either state, accidentally or otherwise.

Tim

[cprobertson1]

I used an HV5122 32 channel open drain driver in my Nixie clock

I hadn't actually realised that family of chips were still in production: I think I'll invest in a few for simplicity's sake. I'll need to etch or buy DIP converter boards from somewhere (I'll try out an etching tonight to see how it goes - toner transfer method doesn't favour fine pitch components I'm afraid!) If that works, score, saves me £1.50 per board

@T3sl4co1l: Thanks! I'll have a play around at some point once I get some parts in. I need to get a high voltage source sorted out as well: I'm thinking a MAX1771 using the application notes for the schematic.

I'll probably look around for a DIP version of that chip as well for the sake of simplicity

Thanks for all your help!

[TimFox]

If you want to go seriously retro (steampunk?), the cathode of a vacuum tube, such as 1/2 12AT7, can be driven from TTL levels if the grid is grounded.  However, the plate will not swing as far negative as a semiconductor.  With a high-mu triode, the TTL level swing should be sufficient to cut off the tube, and the driver must sink the cathode current in the on state.

[T3sl4co1l]

One advantage that you'd have with suitably sized triodes is, the plate resistance naturally limits current.  So you could drive the nixie anode from fixed voltage and use an array of 12AX7, or AT or AU, to drive the cathodes with the required current.  Just a small simplification, but hey, saving resistors and all that...

FWIW, driving the grid positive (say, to VCC) allows pretty reasonable [saturation and current draw] performance from triodes; though you won't get such good cutoff in such a case!

Tim

[TimFox]

Depending on the current you want, use either 12AX7 (very low current) or 12AT7 (medium current).  Their high mu value requires a smaller cutoff voltage.  In the circuit shown, the cutoff bias would be about 5 V, if the driver swings to the +5V rail.  To first order, the cutoff voltage is Vco = Vpk / mu, and the mu of a 12AU7 is only about 20, compared to 60 for the 12AT7.  All three tubes are pin-compatible.

[cprobertson1]

Righty-ho!

Getting near the finalisation of my parts-order: just need to sort out the power supply

I've been hum-ing and ho-ing and to-ing and fro-ing over what to do for the power supply:

-Should I buy one? (Nah! Making one is more fun!)

-Should I step down from Mains AC? (Overkill: plus I was hoping to have it battery powered!)


That leaves DC:DC step-up controllers; stepping up 9V or even 5V USB to the HV required for the Nixies - current draw should be low enough for batteries. 8x Nixies running at max allowable currents is only 24mA (and that's with me being generous and assuming 20% extra per tube on top of the max allowed in the datasheet)

I've seen both the MAX1771ESA and the MC34063AP used in various PSUs.

The MC34063AP has the benefit of being a DIP package which saves me etching my own board (which I wouldn't trust at high voltages as I'm stuck with the toner transfer method!) it's also cheaper than the MAX1771. The datasheet (I believe) says its rated to 40V output: but a number of people appear to have used it so I may be misinterpreting the datasheets!

The MAX1771ESA comes in an SO-8 package - which means I'd need an adapter (which is easy enough to do - I have adaptors for the HV5122 packages anyway so it's not a huge deal: just means I have a few daughter boards kicking about the innards) - these are considerably more expensive than the MC34063, but has a higher voltage rating (I think I saw the max output was 100V? but again, people have driven these at 170 and 180V_DC so I'm again unsure how to interpret the datasheets!

Can anybody advise me further on these matters?

Many thanks!

[aargee]

Personally, I'd stay away from direct mains sourced voltages due to the inherent danger and the pain of wiring for AC.

For chip based power supplies- The reality is, if you're driving your inductor with a MOSFET,like most of the designs out there, you can have any terminal output voltage (within reason) as the chip is not exposed to the o/p voltage directly. Do a search on either chip with the term  'Nixie Power Supply' added and have a look at the large number of designs out there. The MAX1771 is slightly more efficient apparently.

I have a MC34063 design producing 180V quite happily, although this is as far as I've got on my nixie clock. 
I'll be building mine on stripboard to give it a rustic appearance, point to point wiring would be a little too laborious 

Good luck!

[linux-works]

if you buy the russian or ukrainian tubes, often the seller will also have the 74141 style driver chips.  I bought some from a US seller but I could have gotton them overseas, too.  they are equivalent to the US old style chips (TI?) but for the very expensive tubes (I forget the name, but they are the super huge nixie that costs more than $200 for a set of the US chips are supposed to drive them better (something about a blue spot appearing on those nice expensive tubes when using russian driver chips).

if you use the more affordable and smaller nixies, the russian chips are fine.  I have them in my nixie clock and for the past year that its been alive, the chips have not failed me yet

they are still buyable.  not sure for how much longer.  probably a few years, at least (guessing).

[cprobertson1]

I'd stay away from direct mains sourced voltages due to the inherent danger and the pain of wiring for AC.

Agreed!

Mains AC scares me: a) because it can kill me fairly easily and b) because it's AC which I rarely play with.

I have a MC34063 design producing 180V quite happily

I've settled for the MC34063 as the DC/DC controller: still to work out the support circuitry for it - that side of things can wait until later though

they are still buyable.  not sure for how much longer.  probably a few years, at least (guessing).

Ah, and there's the rub! I'm wanting to use (as much as possible) off-the-shelf parts that are still in production or are easily replaceable :p Which is a pity, because some of these old driver chips would have done the business! Though some of them are a more than alittle costly anyway (at least to buy in the UK (at least as far as I've seen!))



And that brings us to today...

I tried to order the bulk of my parts only to discover that the HV5622 I had originally settled on as the nixie driver have a minimum purchase order of 96 from Microchip Direct: and I don't have a spare £400; unavailable from uk.farnell; and I couldn't be bothered transferringmy entire order into digikey. Actually, importing it probably wouldn't be too hard, but I can't be bothered figuring it out.


Great! Now I need another driver solution (I thought I had nailed it with the HV5622 and it's family of chips! I mean, I could get them off ebay, but most looked like they were going to take a month to arrive; and some even said they had been used (wait... what!? ew... second hand chips... yuck...))

One option is to return to the idea of driving transistors from a shift register - and by coincidence I already had a latching shift register picked out for running the LED-based fake-nixie tubes; which would almost certainly do the trick (and the latching makes things simple)

Would I be right in saying that tying the common-anode nixie tubes to the collector on the transistors (and having a resistor on the emitter which is tied to ground) work as a solution?

If so, for the sake of an extra layer of protection, would a high-voltage diode between the shift register and the transistor be effective?


Ps - I'm aware hand-soldering 96 transistors is going to be one of the most wonderful ideas I've ever came up with! But it's going to be done as a modular series of boards so I can bring the workload to a manageable level (and easily replace anything that blows up)

[linux-works]

maybe helpful site: google 'arduinix'.

I bought a board from him, built it but never got around to hooking it up   he has onboard psu, driver transistors and uses 2 russian 74141 chips.  you can drive 4 or 6 or even 8 if you mux them.  he has sample code, too.

might give you ideas.

[cprobertson1]

Ooh! Plenty of ideas kicking about xD

Right: Two questions;

Question 1: So what is the opinion on muxing these? The Datasheet suggests they have a "Multiplex Mode" - but will it increase the chance of them failing? (and can anybody confirm the 1 – 1.8 kHz frequency on the PWM?). I'm aiming for longest possible lifetime on my nixies xD

Question 2: Purely out of curiosity - let's assume I screw up the driving code and accidentally open two cathodes (i.e effectively telling it to display two digits at the same time) - what happens?  I would have liked to have thought the current limiting resistor on the anode would have limited the current so it doesn't power on - but I'm not actually sure!

[SeanB]

The one closest to the anode will invariably win, but if a rear one is already on it will tend to keep the others from having enough voltage to strike.

[T3sl4co1l]

Yeah, the cathodes only have to bob up/down by the difference between lowest glow voltage and highest breakdown voltage.  Something like 30 or 50V, rather than all 140+.

And distance will yield a higher drop, so there's that.

Probably, releasing one cathode while pulling down another (within a few microseconds) will push the plasma around, so that the current flow, and anode voltage, remain fairly constant rather than drooping out as would happen with two completely separate lamps.  That is to say, the presence of a glow on any electrode will encourage any other electrode to discharge, so it doesn't have to switch the full voltage, from fully off (un-ionized) to on.  Which means the change in voltage could be even less.

I wouldn't think multiplex will really gain you much, and you don't have much current capacity and therefore brightness to work with.  They're not like LEDs, which you can pulse well over the static ratings.  Which stinks, because you get the biggest mux savings by using large division ratios.

Tim

[cprobertson1]

@SeanB - so it won't wreck them then; that's good news



Well this afternoon has been spent trying to lay out a PCB to make the soldering easier.

Yuck - layout design problems; my favourite! I wonder if a transistor array would make life easier... except... they don't tend to come in high voltage-tolerant versions do they...?

I'll post up the schematic diagrams once I get one that doesn't look horrific!

[T3sl4co1l]

Well, a 2N3904 style array will already get you more than enough (60Vceo, breakdown around 100V).  If you demand full supply range, there's probably something in the MPSA42 range.  Although, when you're talking 200V on an array, you can't really miniaturize below SOIC level, and even that's pushing it.

Or among ULNxxxx types, there's probably something over 30V too.

Tim

[cprobertson1]

Oops sorry, I meant an integrated array - as opposed to discrete transistors (which is what I was using in the PCB: just a line of them for each Nixie. I was thinking of using the PHD13003C  - and that's where my layout problems were coming from: spec-wise they're overkill for a nixie but fairly cheap and I was already using them for a different project so it wasn't a huge problem to tack an extra... 96 of them onto my order. I am NOT looking forward to soldering that...)

Reckon these might do the trick? (and am I right in saying that it's a simple case of applying current to the base (1B through 7B) and a current will come out of the collector (1C to 7C) based on the Emitter/Common voltage?)

Wait... why does it have 7 transistors and not an even number? Weird xD Might try to find one with 8 transistors to make thing simpler... anyway - will that style of array work if I can get it in a 100V Collector-Emitter Rating?


Out of curiosity - what sort of inductance do nixie tubes have? Would you consider them to be an inductive load (obviously not on the same sort of level as a motor; but worth taking into account?) I couldn't find much data on the inductance of them I'm afraid (though I didn't look for particularly long: too much stuff to do )

[TimFox]

The inductance should be negligible:  they are just a glorified neon bulb or VR tube.  You can see the short leads inside the glass envelope between the base pins and the electrodes.  The main weird thing about their behavior is hysteresis between the "strike" voltage and the "sustain" voltage, but that is a non-linear, non-reactive effect.

[cprobertson1]

Aye, some of their properties are a bit weird My favourite being their negative resistance once struck xD

They're such fascinating little devices!


Now to get back to the drawing board yet again and find yet another driver solution I should also probably get back to work at some point - I'm totally not in the office at the moment.

--EDIT--

That 7-transistor-IC is causing problems... could really do with an 8-transistor version of it! Which... it turns out (at least from Farnell) only come in 50V variants.

Take it a 50V Transistor is too low (bearing in mind a c.170V strike voltage: how much does it drop after it's struck? I thought it was only something like 20-30V drop?)

Ps - I'll do some proper research tonight on the subject - I'm just throwing my thoughts about just now!

[T3sl4co1l]

No need for 500mA darlingtons, probably undesirable in fact, as you have to contend with max 0.5mA collector leakage!

The heptet package is simply what fits in a DIP16.  DIP18?  What's that?

Tim